1. Field of the Invention
The present invention relates to a lightweight bearing, and to a wave gear drive in which a rigid, circular internal gear and a flexible, circular external gear are coupled by the lightweight bearing so that the gears can rotate relative to each other.
2. Description of the Prior Art
In JP-A 09-250608, this applicant proposed a compact wave gear drive unit that uses a cross-roller bearing. The compact wave gear drive unit of this disclosure included first and second end-plates disposed at each end in the unit""s axial direction, a top-hat-shaped wave gear drive incorporated between the end-plates, and a cross-roller bearing that rotatably supports a top-hat-shaped flexible external gear and rigid internal gear constituting the wave gear drive, a drive housing being comprised by an outer ring of the cross-roller bearing and the first and second end-plates. In JP-A 09-303496, this applicant also proposed a wave gear drive unit having a similar structure, that included a cup-shaped wave gear drive.
Such wave gear drive units are compact enough to be used as reduction gear mechanisms of actuators incorporated in robotic arms and other such applications in which installation space is limited. Generally, it is necessary for the constituent components of such mechanisms to be light in weight. In a wave gear drive unit, the cross-roller bearing and rigid internal gear in particular are heavy, so if the weight of these parts could be reduced, it would make it possible to reduce the weight of the wave gear drive unit.
However, the raceway surfaces of the cross-roller bearing and the teeth of the rigid internal gear have to be durable and wear-resistant, which has necessitated the use of heavy iron-based materials. Moreover, the bearing rings of the cross-roller bearing and rigid internal gear are affixed to other members such as the unit housing, so if such members are made of a lightweight material such as aluminum alloy, the members are not able to withstand the seating pressure of the fixing bolts, leading to seating faces that are defective or strained and the like, thereby degrading the fixing force. For these reasons, it has been difficult to reduce the weight of cross-roller bearings and wave gear drive units.
A main object of the present invention is to provide a lightweight cross-roller bearing and wave gear drive in which the light weight is achieved without harming the performance of the bearing raceway surfaces or rigid internal gear teeth while at the same time ensuring the seating-face strength of the fastening surfaces of the bearing rings and rigid internal gear of the wave gear drive.
To attain the above object, the present invention provides a lightweight bearing in which at least an outer or inner bearing ring has fixing holes formed therein for fixing the ring to another member, the bearing ring comprising a main bearing ring member in which said fixing holes are formed, and a raceway surface formation member that is fixed to the main bearing ring member and forms a raceway surface on an inner peripheral surface or outer peripheral surface, the raceway surface formation member is formed of an iron-based material, the main bearing ring member is formed of a lightweight material that is lighter than the iron-based material, and a hard plated coating is formed on surface portions of the main bearing ring member that form a seat for fasteners used in the fixing holes.
The coating can be an electroplated or electroless coating of nickel or chromium, and the coating can be approximately 5 microns thick. The lightweight material can be an alloy of a light metal such as aluminum alloy or titanium alloy, or can be plastic, or ceramics.
When said ring is the outer bearing ring, the main bearing ring member can be formed of aluminum alloy, and the raceway surface formation member comprised of an iron-based material can be cast in the aluminum alloy or inserted into the main bearing ring member, using pressing by shrink-fitting or the like to unite the two members. The main bearing ring member can instead be formed of titanium alloy and forging used to unite the raceway surface formation member formed of iron-based material with the main bearing ring member.
When said ring is the inner bearing ring, the main bearing ring member can be formed of titanium alloy, and the raceway surface formation member comprised of an iron-based material can be cast in the titanium alloy or inserted into the main bearing ring member, using pressing by shrink-fitting or the like to unite the two members. The main bearing ring member can instead be formed of aluminum alloy and forging used to unite the raceway surface formation member formed of iron-based material with the main bearing ring member.
The above and other objects can also be attained by a wave gear drive comprising a rigid internal gear, a flexible external gear located inside the internal gear, a wave generator located therein, and a lightweight bearing having the above-described configuration that couples the rigid internal gear and flexible external gear in a state in which there can be relative rotation between the internal gear and the external gear.
The rigid internal gear comprises a main gear member, and a circular teeth formation member that is attached to an inner peripheral surface of the main gear member and has internal teeth formed on an inner peripheral surface. The teeth formation member is formed of an iron-based material and the main gear member can be formed of a lightweight material that is lighter than the iron-based material.
The wave generator comprises a rigid cam plate and a ball bearing that fits on an outer peripheral surface of the rigid cam plate. The rigid cam plate can be formed of a lightweight material that is lighter than the iron-based material.
When the main gear member of the rigid internal gear has fixing holes formed therein for attachment to another member, it is desirable to form a hard plated coating of a surface portion that seats fasteners used in the fixing holes. The hard plated coating can be an electroplated or electroless coating of nickel or chromium approximately 5 microns thick. It is preferable for the seat to have a limit contact pressure that is at least about 25 kgf/mm2. The lightweight material can be an alloy of a light metal such as aluminum alloy or titanium alloy, or can be plastic or a ceramics material.
In the wave gear drive of the invention thus comprising a rigid internal gear, a flexible external gear located inside the internal gear, a wave generator located therein, and a lightweight bearing configured to couple the rigid internal gear and flexible external gear in a state in which there can be relative rotation between the internal and external gears, the rigid internal gear comprises a main gear member, and a circular teeth formation member that is attached to an inner peripheral surface of the main gear member and has internal teeth formed on an inner peripheral surface, with the teeth formation member being formed of an iron-based material and the main gear member formed of a lightweight material that is lighter than the iron-based material. The main gear member of the rigid internal gear has fixing holes formed therein for attachment to another member, and a surface portion that forms seats of fasteners used in the fixing holes has a hard-plate coating. The hard plated coating can be an electroplated or electroless coating of nickel or chromium approximately 5 microns thick. The fastener seats have a limit contact pressure that is at least about 25 kgf/mm2. The lightweight material can be an alloy of a light metal such as aluminum alloy or titanium alloy, or can be plastic or a ceramics material.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.